1. Field of the Invention
Apparatuses consistent with the present invention relate to a tunable capacitor, and more particularly, to a tunable capacitor using an electrowetting phenomenon, which facilitates a fabrication process, provides good reliability and durability, and has no limitation on a tuning range.
2. Description of the Related Art
The present-day radio portable communication is advancing to multipurpose electronic devices that cover various functions such as camera, game, music play, broadcast, and Internet, beyond the basic telephone function and the messaging function. To service those various functions, it is required to transmit radio frequency (RF) data containing various additional information corresponding to the functions. Currently, a variety of frequency bands and communication protocols are being used for the RF data transmission. Particularly, fabrication of a reconfigurable RF transceiver, which can operate in the multibands and multistandards to meet a user's requirements, is necessary so as to integrate various communication standards into a handheld portable terminal. A tunable matching network using a tunable capacitor and a tunable inductor is requisite to this end.
Diverse techniques have been developed and utilized to fabricate a low and high frequency tunable capacitor. Specifically, the fabrication of a small tunable capacitor mostly adopts microelectromechanical systems (MEMS) technique, which can be divided according to a driving scheme into material property tuning (ferroelectric materials), electrostatic actuation, piezoelectric actuation, thermal actuation, electromagnetic actuation, electrodynamic actuation, and so forth. While using the diverse driving schemes, capacitance C to be ultimately controlled is expressed as a simple equation as below:C=∈·A/d where ∈ is a dielectric constant of a dielectric material, A is an area of parallel electrodes, and d is a distance between the parallel electrodes. Except in a case where the dielectric constant ∈ of the dielectric material is changed, when tuning by varying the area A or the distance d, movement of a microstructure is requisite.
FIG. 1 depicts a dual-type tunable capacitor which varies the distance d between the parallel electrodes using the electrostatic actuation, for example of the MEMS tunable capacitor. Unfortunately, since the MEMS tunable capacitor, like many other MEMS elements, has a movable substance, its fabrication is complicated. Even after the fabrication, fatigue by the repetitive operations obstructs the durability and the reliability. In addition, the movable substance supported by a spring is vulnerable to impacts.
To tune the capacitance by varying the dielectric constant E of the dielectric material, a ferroelectric material such as Barium Strontium Titanate (BST) is used. Alternatively, two unmixable fluids having different dielectric constants are inserted into the microchannel including the parallel electrodes and then tuned by moving them. The former method takes advantage of a property that the ferroelectric material becomes a paraelectric material above the Curie temperature Tc and that the dielectric constant of the paraelectric material is changed by an external voltage. The BST allows controlling Tc to the room temperature and provides high dielectric constant in the microwave frequency and low loss, whereas it is needed to ensure the stable property of matter in view of the material. The latter method achieves the tuning by inserting two unmixable nonconductive materials between the fixed parallel electrodes and adjusting the quantity of the fluids between the electrodes. The greater difference between the dielectric constants of the two fluids, the larger tuning range. Since the nonconductive fluid, rather than the solid structure, is movable, the durability and the reliability can be achieved for a long term but another driving method is required to move the fluids.